1. Technical Field
The present invention relates to a power supply unit, and more specifically, to a boosting power supply unit and a power boosting control method.
2. Discussion of the Related Art
In general, thin film transistors (TFTs) in liquid crystal panels having a turn on voltage of about 20V and a turn off voltage of about −20V are used to drive TFT liquid crystal display devices (LCDs), such as those for use with mobile video graphic adapters (VGA), etc. Such TFT turn-on/off voltages have slight deviations depending on the types of TFTs (for example, amorphous silicon (a-Si) TFTs, low-temperature polysilicon (LTPS) TFTs, continuous grain silicon (CGS) TFTs, etc.) used in the liquid crystal panels, and the sizes of the liquid crystal panels. Because the voltages applied from battery powered sources of mobile devices, such as mobile TFT LCDs, are generally about 3V, boosting circuits for stepping up the voltages from 3V to 20V or stepping down the voltages from 3V to −20V are required to drive the mobile TFT LCDs.
U.S. Pat. No. 5,461,557 discloses a conventional boosting circuit 100 as shown in FIG. 1.
Referring to FIG. 1, the conventional boosting circuit 100 comprises ten switches SW1 through SW10, and four capacitors Ca through Cd, and generates a boosted positive voltage 3VDD, which is three times a source voltage VDD, by stepping up the source voltage VDD, and a boosted negative voltage −2VDD, which is two times the source voltage VDD, by stepping down the source voltage VDD. For example, as shown in FIG. 1, the source voltage VDD is applied to a first capacitor Ca and a second capacitor Cb through the switches SW1 through SW4, which are closed in response to a first clock signal P1. The boosted positive voltage 3VDD is successively output from a third capacitor Cc through the switches SW5 through SW7, which are closed in response to a second clock signal P2. Similarly, the source voltage VDD is applied to the first capacitor Ca and the second capacitor Cb, and then the boosted negative voltage −2VDD is output from a fourth capacitor Cd through the switches SW8 through SW10, which are closed in response to a third clock signal P3. The stepped-up and stepped-down voltages (e.g., 3VDD and −2VDD, respectively) generated by the conventional boosting circuit 100 are used as a power source to drive gates of the TFTs, for example, TFTs in the liquid crystal panel of LCDs, etc., and thus turn the TFTs on or off. In addition, the stepped-up and stepped-down voltages can be used for circuits, which need to obtain high voltages from low voltages, and circuits, which need to obtain low voltages from high voltages, etc.
Although the conventional boosting circuit 100 can output the boosted positive voltage 3VDD and the boosted negative voltage −2VDD under two-phase driving, the source voltage VDD applied to the capacitors Ca through Cd is constant, so that a boosting efficiency of the conventional boosting circuit 100 is low. Further, the conventional boosting circuit 100 cannot generate a variety of boosted voltages, such as boosted voltages, which are four times and six times the source voltage VDD, etc. In addition, because conventional power supply units comprising the conventional boosting circuit 100 output the boosted voltages without regard to the amount of load power there is a large amount of power consumption.